Some MOS integrated circuits have several types of MOS transistors that operate at different voltages. For example, an EEPROM may have high voltage MOS transistors for programming of the device and low voltage transistors making up the memory cells of the device. Another example of integrated circuits with both high and low voltage devices are integrated circuits designed to be compatible with older technologies. As the device size in MOS integrated circuits shrinks, the oxide thickness and operating voltage of MOS transistors internal to the device are reduced. To retain compatibility with older technologies, however, higher voltage transistors with thicker oxides may be used for input and/or output circuitry of the integrated circuit.
MOS integrated circuits having transistors with different operating voltages present certain problems in fabrication. To accomplish a variance in operating voltages, different transistors are fabricated with different gate oxide thicknesses. Higher voltage transistors have a thicker gate oxide. In a commonly used existing process, a gate oxide layer is initially formed, and then patterned and removed from the region of the integrated circuit that will have low voltage MOS transistors. The subsequent removal of photoresist that was used to mask and protect the area of the integrated circuit having high voltage transistors exposes the wafer (in many cases bare silicon) in the low voltage region to impurities in the photoresist and the chemicals used for removal of the photoresist. In addition, the photoresist cleanup results in gate oxide integrity problems in the high voltage region.